What is the effect of altitude and pressure on the power handling of a coaxial cable?

Altitude and atmospheric pressure significantly affect the power handling capability of coaxial cables and connectors. At higher altitudes (lower air pressure), the reduced air density decreases the dielectric strength of air, making voltage breakdown (arcing) more likely: (1) Paschen law: the breakdown voltage of a gas gap depends on the product of pressure and gap distance (p × d). As altitude increases and pressure decreases: the breakdown voltage decreases. At sea level (1 atm = 101.3 kPa): air breakdown occurs at approximately 3 kV/mm (for uniform fields). At 3,000 m (10,000 ft, 70 kPa): breakdown voltage drops to approximately 70% of sea level. At 10,000 m (33,000 ft, 26 kPa): breakdown voltage drops to approximately 30-40% of sea level. At 20,000 m (65,000 ft, 5.5 kPa): breakdown voltage drops to approximately 10-15% of sea level. (2) Effect on coaxial cable: the peak voltage in a coaxial cable is: V_peak = sqrt(2 × P × Z0). For 100 W into 50 ohms: V_peak = sqrt(2 × 100 × 50) = 100 V. For 1 kW into 50 ohms: V_peak = 316 V. At altitude: the reduced breakdown voltage means that the cable may arc at power levels that are safe at sea level. The critically affected areas: connectors (where the dielectric gap is smallest and field concentration is highest), adapter junctions, and any point where air is present in the dielectric path. (3) Derating: cable manufacturers provide altitude derating curves. Typical derating factors: at 3,000 m: derate power to 75% of sea level rating. At 6,000 m: derate to 50%. At 10,000 m: derate to 25-35%. For pressurized cables (cables filled with dry nitrogen or SF6 gas): the internal pressure is maintained regardless of altitude, so no derating is needed. Pressurized cables are standard for high-power transmitter installations, especially at mountain-top sites (broadcast towers) and airborne radar systems. (4) Other altitude effects: temperature: ambient temperature decreases approximately 6.5°C per 1,000 m of altitude gain. Lower temperature improves cable loss and power handling, partially offsetting the pressure effect. Humidity: decreases at altitude (dry air at high altitude). Lower humidity generally improves voltage breakdown (water vapor contributes to breakdown).